This invention relates to interferometers, e.g., interferometers for measuring the position and orientation of a measurement object such as a mask stage or a wafer stage in a lithography scanner or stepper system.
Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beam components that enter the interferometer have orthogonal polarizations and frequencies separated by a heterodyne, split-frequency. The split-frequency can be produced, e.g., by Zeeman splitting, by acousto-optical modulation, or by positioning a birefringent element internal to the laser. A polarizing beam splitter directs the measurement beam along a measurement path contacting a reflective measurement object (e.g., a stage mirror), directs the reference beam along a reference path, and thereafter recombines the beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that passes through a polarizer that mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a xe2x80x9cheterodynexe2x80x9d signal at a split-frequency. When the measurement object is moving, e.g., by translating a reflective stage, the heterodyne signal is at a frequency equal to the split frequency plus a Doppler shift. The Doppler shift equals 2 vp/xcex, where v is the relative velocity of the measurement and reference objects, xcex is the wavelength of the measurement and reference beams, and p is the number of passes to the reference and measurement objects.
Changes in the optical path length to the measurement object correspond to changes in the phase of the measured interference signal at the split-frequency of the measurement and reference beam components that enter the interferometer, with a 2xcfx80 phase change substantially equal to an optical path length change nL of xcex/p, where n is the average refractive index of the medium through which the light beams travel, e.g., air or vacuum, and where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object. Similarly, multiple interferometers can be used to measure changes in distance to multiple points on the measurement object from which changes in the angular orientation of the measurement object can be determined.
Such interferometers are often crucial components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. The lithography systems typically include: at least one movable stage to support, orient, and fix the wafer; focusing optics used to direct a radiation beam onto the wafer; a scanner or stepper system for translating the stage relative to the exposure beam; and one or more interferometers to accurately measure changes in the position of the stage relative to the radiation beam. The interferometers enable the lithography system to precisely control which regions of the wafer are exposed to the radiation beam.
The invention features an interferometry system for use in measuring changes in any of an angular orientation of a measurement object, a position of a measurement object, an optical distance to the measurement object, a physical distance to the measurement object, and a dispersion in the path to the measurement object. The interferometry system includes at least one dynamic beam-steering assembly and one or more optical fibers to couple beams produced by the interferometry optics to remote photodetectors and associated electronics.
The photodetectors and associated electronics can generate significant heat during operation, which can adversely affect the interferometer optics and degrade interferometric measurements. Using optical fibers to remotely position such detection electronics reduces the heat load on the environment of the interferometer. In addition, the remote positioning decreases the sensitivity of the detector to any electrostatic discharge (ESD) and radio frequency interference (RFI) present in the interferometer environment. For example, ESD and RFI can arise when the interferometer is coupled to a microlithography stage that rapidly repositions a wafer stage or reticle.
But, problems can arise when coupling an interferometric output beam into an optical fiber. For example, in many conventional interferometers, changes in the angular orientation or position of the measurement object typically introduce a lateral displacement (or xe2x80x9cshearxe2x80x9d) between the interfering components of the output beam. Such lateral displacement reduces coupling efficiency into the fiber, and thereby reduces the interference signal measured by a subsequent detector. Furthermore, when using a multimode fiber, the lateral displacement can cause the interfering components of the output beam to couple into different spatial modes of the multimode fiber. As the components propagate to the end of the fiber, modal dispersion introduces a phase shift between them. Because the phase shift varies with the lateral displacement (which in turn varies with the angular orientation or position of the measurement object), it corrupts the interferometric measurement associated with the measurement object.
Fortunately, the interferometry systems disclosed herein, which include one or more dynamic beam steering assemblies, greatly reduce the lateral displacement between the interfering components of the interferometric output beam, and thereby mitigate the problems of fiber optic coupling efficiency and modal dispersion.
The dynamic beam-steering assembly redirects one or more beams within the interferometry system in response to a change in the angular orientation or position of the measurement object. Interferometry systems employing a dynamic beam steering assembly are also disclosed in the following commonly owned applications, all of which are incorporated herein by reference: U.S. application Ser. No. 09/157,131, filed Sep. 18, 1998 entitled INTERFEROMETER HAVING A DYNAMIC BEAM STEERING ASSEMBLY by inventors Henry A. Hill and Peter de Groot; U.S. application Ser. No. 09/305,876, filed May 5, 1999 entitled SINGLE-PASS AND MULTI-PASS INTERFEROMETRY SYSTEMS HAVING A DYNAMIC BEAM-STEERING ASSEMBLY FOR MEASURING DISTANCE, ANGLE, AND DISPERSION by inventors Henry A. Hill and Peter de Groot; U.S. application Ser. No. 09/305,828, filed May 5, 1999 entitled INTERFEROMETRY SYSTEM HAVING A DYNAMIC BEAM STEERING ASSEMBLY FOR MEASURING ANGLE AND DISTANCE by inventor Henry A. Hill; U.S. application Ser. No. 09/384,851, filed Aug. 27, 1999 entitled INTERFEROMETRY SYSTEM HAVING A DYNAMIC BEAM STEERING ASSEMBLY FOR MEASURING ANGLE AND DISTANCE by inventor Henry A. Hill; and International Application Serial No. PCT/US00/12097, filed May 5, 2000, entitled INTERFEROMETRY SYSTEM HAVING A DYNAMIC BEAM-STEERING ASSEMBLY FOR MEASURING ANGLE AND DISTANCE by inventor Henry A. Hill. In addition to the interferometry systems explicitly disclosed herein, any of the interferometry systems disclosed in the applications referenced above can modified to include one or more optical fibers to provide remote photoelectric detection.
In general, in one aspect, the invention features an interferometry system including: an interferometer which during operation directs a measurement beam along a measurement path contacting a measurement object and combines at least a portion of the measurement beam with another beam to form an overlapping pair of exit beams, the interferometer including a beam steering assembly having a beam steering element and a positioning system to orient the beam steering element, the beam steering element positioned to direct the measurement beam, the measurement beam contacting the beam steering element; a control circuit which during operation causes the positioning system to reorient the beam steering element in response to a change in at least one of angular orientation and position of the measurement object; a photodetector; and an optical fiber coupling an output beam derived from the overlapping pair of exit beams from the interferometer to the photodetector.
In general, in another aspect, the invention features an interferometry system including: an interferometer which during operation receives an input beam, splits the input beam into a measurement beam and at least one other beam, directs the measurement beam along a measurement path contacting a measurement object, and combines at least a portion of the measurement beam with the other beam to form an overlapping pair of exit beams; a beam steering assembly having a beam steering element and a positioning system to orient the beam steering element, the beam steering element positioned to direct the input beam, the input beam contacting the beam steering element; a control circuit which during operation causes the positioning system to reorient the beam steering element in response to a change in at least one of angular orientation and position of the measurement object; a photodetector; and an optical fiber coupling an output beam derived from the overlapping pair of exit beams from the interferometer to the photodetector.
Embodiments of either of the interferometry systems employing an optical fiber described above can include any of the following features.
The optical fiber can be a multimode fiber. The system can further include a polarizer positioned to receive at least a portion of the overlapping pair of exit beams from the interferometer and produce the output beam by mixing polarization components of the at least a portion of the overlapping pair of exit beams. The interferometry system can further include a signal processor coupled to the photodetector which during operation determines a change in the at least one of the angular orientation and the position of the measurement object based on interferometric signals measured by the photodetector. The interferometry system can further include a signal processor coupled to the photodetector which during operation determines a change in at least one of an optical path length to the measurement object and a physical path length to the measurement object based on interferometric signals measured by the photodetector. The interferometry system can further include an angle measuring system which during operation calculates the change in at least one of the angular orientation and the position of the measurement object based on the reorientation of the beam steering element. The photodetector can be part of the control circuit, or alternatively, it can be separate from the control circuit.
In embodiments in which the first-mentioned photodetector is separate from the control circuit, the control circuit can include a second photodetector, and the interferometry system can further include a second optical fiber for coupling a signal beam derived from the overlapping pair of exit beams to the second photodetector. A polarizer can be positioned to receive at least a portion of the overlapping pair of exit beams from the interferometer and produce the signal beam by mixing polarization components of the at least a portion of the overlapping pair of exit beams. In addition, the interferometry system can include an etalon and at least a portion of the overlapping pair of exit beams can be incident on the etalon prior to contacting the polarizer.
Also, in embodiments in which the first-mentioned photodetector is separate from the control circuit, the control circuit can include a second photodetector, and the interferometry system can further include a second optical fiber for coupling a signal beam derived from the measurement beam to the second photodetector. The interferometry system can further include an etalon, wherein a beam derived from the measurement beam is incident on the etalon to produce the signal beam.
Further, in embodiments in which the first-mentioned photodetector is separate from the control circuit, the control circuit can include a detector having spatially resolved detector elements operative to measure the position and/or direction of at least a portion of the overlapping pair of exit beams, and the interferometry system includes a second optical fiber for coupling the at least a portion of the overlapping pair of exit beams from the interferometer to the second detector.
The additional beam in the interferometer can be a second measurement beam contacting the measurement object at a location spatially separated from the first mentioned measurement beam. Alternatively, the additional beam is a reference beam that does not contact the measurement object. Also the interferometer can separate the measurement beam into at least two portions, the other beam being derived from one of the portions.
Additional aspects of the invention follow.
In general, in one aspect, the invention features an interferometry system including an interferometer, a control circuit, and an angle measurement system. During operation the interferometer directs a measurement beam along a measurement path contacting a measurement object and combines each of at least two portions of the measurement beam with a corresponding reference beam to form at least two overlapping pairs of exit beams. The interferometer includes a beam-steering assembly having a beam-steering element and a positioning system to orient the beam-steering element, the beam-steering element positioned to direct the measurement beam to the measurement object and the measurement beam contacting the beam-steering element. During operation the control circuit causes the positioning system to reorient the beam-steering element in response to a change in angular orientation of the measurement object and the angle measurement system which calculates the change in angular orientation of the measurement object based on at least one of interferometric signals derived from the overlapping pairs of exit beams and the reorientation of the beam-steering element.
In some embodiments the interferometer directs the at least two portions of the measurement beam to contact the beam-steering element before combining each of them with the corresponding reference beam. In other embodiments, the interferometer directs the measurement beam to contact the beam-steering element a second time, after which the interferometer separates the measurement beam into the at least two portions.
In further embodiments, the interferometer further includes a plurality of reflective surfaces oriented to direct the measurement beam, the portions of the measurement beam, a progenitor beam for the reference beams, and the reference beams. For initial linear polarizations and propagation directions for the measurement beam and the progenitor beam, the plurality of reflective surfaces can be oriented to preserve a linear polarization for the measurement beam, the portions of the measurement beam, a progenitor beam for the reference beams, and the reference beams upon their successive reflections.
In general, in another aspect, the invention features an interferometry system including an interferometer, a control circuit, and an angle measurement system. During operation the interferometer directs a measurement beam along a measurement path contacting a measurement object, separates the measurement beam into m portions, e.g., where m is one of 2 and 3, and recombines at least a part of one of the portions with each of the remaining mxe2x88x921 portions to form mxe2x88x921 overlapping pairs of exit beams. The interferometer includes a beam-steering assembly having a beam-steering element and a positioning system to orient the beam-steering element. The beam-steering element is positioned to direct the measurement beam to the measurement object and receive the m separated portions, the measurement beam and each of the m separated portions contacting the beam-steering element. During operation the control circuit causes the positioning system to reorient the beam-steering element in response to a change in angular orientation of the measurement object, and the angle measurement system calculates the change in angular orientation of the measurement object based on at least one of interferometric signals derived from the mxe2x88x921 overlapping pairs of exit beams and the reorientation of the beam-steering element.
In some embodiments, the interferometer further includes a plurality of reflective surfaces oriented to direct the measurement beam and the portions of the measurement beam. For an initial linear polarization and propagation direction for the measurement beam, the plurality of reflective surfaces can be oriented to preserve a linear polarization for the measurement beam and the portions of the measurement beam upon their successive reflections.
In general, in another aspect, the invention features an interferometry system including an interferometer, a control circuit, and an angle measurement system. During operation the interferometer directs two measurement beams along corresponding measurement paths contacting a measurement object and combines the measurement beam to form an overlapping pair of exit beams. The interferometer includes a beam-steering assembly having a beam-steering element and a positioning system to orient the beam-steering element. The beam-steering element is positioned to direct the two measurement beams to the measurement object, the two measurement beams contacting the beam-steering element. During operation the control circuit causes the positioning system to reorient the beam-steering element in response to a change in angular orientation of the measurement object and the angle measurement system calculates the change in angular orientation of the measurement object based on at least one of an interferometric signal derived from the overlapping pair of exit beams and the reorientation of the beam-steering element. In some embodiments, the two measurement beams contact the measurement object at substantially the same location.
In some embodiments, the interferometer further includes a plurality of reflective surfaces oriented to direct the measurement beams. For an initial linear polarization and propagation direction for a progenitor beam for the measurement beams, the plurality of reflective surfaces can be oriented to preserve a linear polarization for the measurement beams upon their successive reflections.
In general, in another aspect, the invention features an interferometry system including an interferometer, a beam-steering assembly, a control circuit, and an angle measurement system. During operation the interferometer receives an input beam, splits the input beam into a measurement beam and m reference beams, where m is an integer greater than 1, directs the measurement beam along a measurement path contacting a measurement object, and combines each of m portions of the measurement beam with a corresponding one of the m reference beams to form m overlapping pairs of exit beams. The beam-steering assembly has a beam-steering element and a positioning system to orient the beam-steering element. The beam-steering element is positioned to direct the input beam and the m overlapping pairs of exit beams, the input beam and the m overlapping pairs of exit beams contacting the beam-steering element. During operation, the control circuit causes the positioning system to reorient the beam-steering element in response to a change in angular orientation of the measurement object and the angle measurement system calculates the change in angular orientation of the measurement object based on at least one of interferometric signals derived from the m overlapping pairs of exit beams and the reorientation of the beam-steering element.
In some embodiments, the interferometer further includes a plurality of reflective surfaces oriented to direct the measurement beam and the reference beams. For an initial linear polarization and propagation direction for the input beam, the plurality of reflective surfaces can be oriented to preserve a linear polarization for the measurement beam and the reference beams upon their successive reflections.
In general, in another aspect, the invention features an interferometry system including an interferometer, a beam-steering assembly, a control circuit, and an angle measurement system. During operation the interferometer receives an input beam, splits the input beam into two measurement beams, directs the measurement beams along respective measurement paths contacting the measurement object, and combines the measurement beams to form an overlapping pair of exit beams. The beam-steering assembly has a beam-steering element and a positioning system to orient the beam-steering element. The beam-steering element is positioned to direct the input beam and the overlapping pair of exit beams, the input beam and the overlapping pair of exit beams contacting the beam-steering element. During operation, the control circuit causes the positioning system to reorient the beam-steering element in response to a change in angular orientation of the measurement object and the angle measurement system calculates the change in angular orientation of the measurement object based on at least one of an interferometric signal derived from the overlapping pair of exit beams and the reorientation of the beam-steering element.
In some embodiments, the interferometer further includes a plurality of reflective surfaces oriented to direct the measurement beams. For an initial linear polarization and propagation direction for the input beam, the plurality of reflective surfaces can be oriented to preserve a linear polarization for the measurement beams upon their successive reflections.
In general, in another aspect, the invention features an interferometry system including an interferometer, a control circuit, and an angle measurement system. During operation, the interferometer directs a measurement beam along a measurement path contacting a measurement object, separates the measurement beam into first and second portions, directs the first and second portions along separate paths, and subsequently recombines the first and second portions with each other to form at least one overlapping pair of exit beams. The interferometer includes a beam-steering assembly having a beam-steering element and a positioning system to orient the beam-steering element, the beam-steering element being positioned to direct the measurement beam to the measurement object and subsequently receive the measurement beam from the measurement object. The measurement beam thereby twice contacts the beam-steering element, after which the interferometer separates the measurement beam into the first and second portions. During operation, the control circuit causes the positioning system to reorient the beam-steering element in response to a change in angular orientation of the measurement object based on at least one interferometric signal derived from the at least one overlapping pair of exit beams. During operation, the angle measurement system calculates the change in angular orientation of the measurement object based on the reorientation of the beam-steering element.
In some embodiments, the interferometer recombines the first and second portions into two overlapping pairs of exit beams, and the at least one interferometric signal is at least two interferometric signals derived from the two overlapping pairs of exit beams. The control circuit includes two detection channels which during operation measure the at least two interferometric signals, one of the detection channels including a quarter wave plate oriented to cause the two interferometric signals to be in quadrature with one another.
Also, in some embodiments, the interferometer separates the measurement beam into third and fourth portions after the measurement beam twice contacts the beam-steering element, directs the third and fourth portions along separate paths, and recombines the third and fourth portions to form a second at least one pair of overlapping exit beams. During operation the control circuit causes the positioning system to reorient the beam-steering element along two dimensions based on the at least one interferometric signal and a second at least one interferometric signal derived from the second at least pair of overlapping exit beams.
Furthermore, in some embodiments, the interferometer can further separate the measurement beam into an additional portion after the measurement beam twice contacts the beam-steering element and combines the additional portion with a reference beam to form an additional pair of overlapping exit beams. The interferometry system further includes a distance measurement system, which during operation measures changes in distance to the measurement object based on an interferometric signal derived from the additional pair of overlapping exit beams.
Also, in some embodiments, the interferometer further includes a plurality of reflective surfaces oriented to direct the measurement beam and the portions of the measurement beam. For an initial linear polarization and propagation direction for the measurement beam, the plurality of reflective surfaces can be oriented to preserve a linear polarization for the measurement beam and the portions of the measurement beam upon their successive reflections.
In general, in another aspect, the invention features an interferometry system comprising an interferometer and a control circuit. During operation the interferometer directs a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object and combines the reference and measurement beams to produce overlapping exit reference and measurement beams. The overlapping exit reference and measurement beams are indicative of changes in a relative optical path length between the reference and measurement paths. The interferometer includes a beam steering assembly having a beam steering element and a positioning system to orient the beam steering element. The beam steering element has at least two faces positioned to direct the measurement beam after it contacts the measurement object, and the measurement beam contacts the measurement object and subsequently contacts each of the two faces during propagation within the interferometer. During operation, the control circuit causes the positioning system to reorient the beam steering element in response to changes in at least one of angular orientation and position of the measurement object. In further embodiments, the system can include an angle measurement system for determining changes in the angular orientation of the measurement object.
Embodiments of any of the interferometry systems described above can include any of the following features.
The angle measurement system can calculate the change in angular orientation of the measurement object based on the reorientation of the beam-steering element or it can calculate the change in angular orientation of the measurement object based on the interferometric signal(s) derived from the overlapping pair(s) of exit beams. The angle measurement system can calculate the change in angular orientation of the measurement object along two dimensions. The distance measurement system can further calculate changes in distance to the measurement object based on at least one of the interferometric signal(s) derived from the overlapping pair(s) of exit beams. The control circuit can cause the measurement beam to contact the measurement object at substantially normal incidence over a range of angular orientations of the measurement object. The control circuit can generate a servo signal based on the interferometric signal(s) derived from the overlapping pair(s) of exit beams and can cause the positioning system to reorient the beam-steering element (e.g., along one or two dimensions) in response to the change in angular orientation of the measurement object based on the servo signal.
Alternatively, the control circuit can include a detector having spatially resolved detector elements operative to measure the position and/or direction of at least a portion of the overlapping pair(s) of exit beams. The control signal generates a servo signal based on the measured position and/or direction and causes the positioning system to reorient the beam-steering element (e.g., along one or two dimensions) in response to the change in angular orientation of the measurement object based on the servo signal. The detector can be operative to measure the difference in position and/or direction between the exit beams of at least one of the overlapping pair(s) of exit beams. Alternatively, the detector can be operative to measure the position and/or direction of the measurement beam component of at least one of the overlapping pair(s) of exit beams relative to a reference position and/or direction.
The invention also features a lithography system for fabricating integrated circuits including first and second components, which are movable relative to one another. One of interferometry systems described above is secured to the second component and the measurement object is a mirror rigidly secured to the first component. During operation the interferometry system measures the position of the first component relative to the second component. In some embodiments, the second component is a movable stage used to support a wafer and during operation the beam-steering element causes the measurement beam to contact the mirror at substantially normal incidence over a range of angular orientations of the measurement object.
In another aspect, the invention also features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes: a stage for supporting the wafer; an illumination system for imaging spatially patterned radiation onto the wafer; a positioning system for adjusting the position of the stage relative to the imaged radiation; and at least one of any of the interferometry systems described above. The interferometry system(s) measures the position of the wafer relative to the imaged radiation.
In another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes a stage for supporting the wafer and an illumination system. The illumination system includes a radiation source, a mask, a positioning system, a lens assembly, and at least one of any of the interferometry systems described above. During operation the source directs radiation through the mask to produce spatially patterned radiation. The positioning system adjusts the position of the mask relative to the radiation from the source. The lens assembly images the spatially patterned radiation onto the wafer. The interferometry system(s) measures the position of the mask relative to the radiation from the source.
In another aspect, the invention features a lithography system for fabricating integrated circuits. The lithography system includes first and second components, the first and second components being movable relative to each other. The lithography system also includes at least one of any of the interferometry systems described above, wherein the first component includes the measurement object and the interferometry system(s) monitors the position of the first component relative to the second component.
In another aspect, the invention features a beam writing system for use in fabricating a lithography mask. The beam writing system includes: a source providing a write beam to pattern a substrate; a stage supporting the substrate; a beam directing assembly for delivering the write beam to the substrate; a positioning system for positioning the stage and beam directing assembly relative one another; and at least one of any of the inteferometry systems described above for measuring the position of the stage relative to the beam directing assembly.
In general, in another aspect, the invention features an interferometry method including: directing a measurement beam along a measurement path contacting a measurement object; combining at least a portion of the measurement beam with another beam to form an overlapping pair of exit beams; using an electronic control system to redirect the measurement beam in response to a change in the angular orientation of the measurement object based on a servo signal derived from at least a portion of the overlapping pair of exit beams; and calculating the change in angular orientation based on an interferometric signal derived from the overlapping pair of exit beams and the redirection of the measurement object. In some embodiments, using the electronic control system includes redirecting the measurement beam to contact the measurement object at substantially normal incidence over a range of angular orientations of the measurement object.
In another aspect, the invention features a lithography method for use in fabricating integrated circuits. The lithography method includes: imaging spatially patterned radiation onto a wafer; positioning the wafer relative to the imaged radiation; and measuring the position of the wafer relative to the imaged radiation using at least one of the interferometry methods described above.
In another aspect, the invention features a lithography method for use in the fabrication of integrated circuits. The lithography method includes: directing input radiation through a mask to produce spatially patterned radiation; positioning the mask relative to the input radiation; measuring the position of the mask relative to the input radiation using at least one of the interferometry methods described above; and imaging the spatially patterned radiation onto a wafer.
In another aspect, the invention features a lithography method for use in fabricating integrated chips. The lithography method includes: positioning a first component of a lithography system relative to a second component of a lithography system to expose a wafer to spatially patterned radiation; and measuring the position of the first component relative to the second component using any of the interferometry methods described above.
In another aspect, the invention features a beam writing method for use in fabricating a lithography mask. The method includes: directing a write beam to a substrate to pattern the substrate; positioning the substrate relative to the write beam; and measuring the position of the substrate relative to the write beam using any of the interferometry methods described above.
In another aspect, the invention features a method for correcting an angle xcex8l indicative of a relative angular orientation of a measurement object for the effects of dispersion caused by gas along a measurement path having a substantially uniform composition, wherein the angle xcex8l is measured interferometrically at a wavelength xcexl. The method includes: i) interferometrically measuring the angular orientation at a first wavelength xcexq to give a first angle xcex8q indicative of the angular orientation; ii) interferometrically measuring the angular orientation at a second wavelength xcexu not equal to the first wavelength xcexq to give a second angle xcex8u indicative of the angular orientation; and iii) correcting the angle xcex8l by an additive factor xcex94xcex8l=xe2x88x92xcex93(xcex8qxe2x88x92xcex8u), where xcex93=(nlxe2x88x921)/(nqxe2x88x92nu) is the reciprocal dispersive power of the gas and nl, nq, and nu are the indices of refraction of the gas at wavelengths xcexl, xcexq, and xcexu respectively. Depending on the embodiment, the angle xcex8l is the first measured angle xcex8q and xcexl=xcexq, the angle xcex8l is the second measured angle xcex8u and xcexl=xcexu, or the angle xcex8l is different from either the first measured angle xcex8q or the second measured angle xcex8u, with xcexlxe2x89xa0xcexq and xcexlxe2x89xa0xcexu.
Embodiments of the interferometry systems and methods described above include many advantages.
Many of the interferometry systems can measure changes in the angular orientation of a measurement object. Furthermore, some embodiments can measure changes in the angular orientation along two dimensions and/or can also measure changes in distance to the measurement object. In addition, many embodiments measure the changes in angular orientation with only a single beam contacting the measurement object or with multiple beams contacting the measurement object at a single location. Thus, the reflective surface of the measurement object can be relatively small. In particular, some of interferometry systems can measure changes in displacement, pitch, and yaw of a stage mirror with a footprint on the stage mirror of only a single beam spot. Moreover, when the measurement beam includes multiple wavelengths, the effects of dispersion along the single beam path to the measurement object can be measured and used to calculate values for displacement, pitch, and/or yaw that are corrected for air-turbulence.
Also, for each of the overlapping pairs of exit beams the interferometry system maintains the overlap of the respective exit beams substantially parallel to one another over a range of orientations of the measurement object, and can do so with only a single pass of the measurement beam to the measurement object. The single-pass embodiments reduce the bandwidth of electronics needed to process electrical interference signals having Doppler shifts, relative to those of a double-pass interferometer. Furthermore, the single-pass embodiments reduce the likelihood of depolarization, scattering, and undesired spurious reflections from transmissive optics within the interferometer, relative to those for a double-pass interferometer. Such effects can introduce errors, e.g., cyclic errors, in the measured phase of the electrical interference signal.
Moreover, the system can minimize the transverse displacement between the exit beams of each overlapping pair of exit beams or the transverse displacement of components of such exit beams within the interferometer, caused by changes in the angular orientation of the measurement object. As a result, the average amplitude of the electrical interference signal derived from optically mixed exit beams can be substantially independent of changes in the angular orientation and position of the measurement object. In addition, the system reduces changes of the paths of the beams through transmissive optics of the interferometer, e.g., polarizing beam splitters and quarter wave plates, caused by changes in the angular orientation or position of the measurement object. Such transmissive optics can have imperfections in their surface figures and local variations in refractive index. Thus, changes in the paths of the beams through such optics can change the optical path length measured by the interferometer even though the distance between the measurement object and the interferometer has not changed. Such negative effects can be compounded if a transmissive optic has dispersive properties, such as those produced by a wedge. Finally, some embodiments of the invention can be made in compact configurations and include optical configurations that minimize the introduction of ellipticity into the linear polarization of beams directed by the optical configuration.
Other features, aspects, and advantages will be clear from the following detailed description and from the claims.